1. Field of the Invention
The present invention relates to an LED head suitably applicable to the formation of an image in combination with the electrophotography and, more particularly, to an LED head with high resolving power and an image forming apparatus such as an LED printer or the like using the LED head. Further, the invention concerns a method of measuring an amount of light from the LED array, for measuring emission characteristics of the LED array used in LED heads and LED printers.
2. Related Background Art
(Prior Art 1)
It is common practice heretofore to use the LED printers with relatively low resolution, e.g., 300 dpi in combination with a bright array of two lines of rod lenses having the nominal angular aperture of 20xc2x0 and the nominal line size of 0.9 mm or 1.1 mm. Using this rod lens array, a photosensitive body is exposed to an emission pattern of LEDs whereby an electrostatic image is formed on the photosensitive body. This electrostatic image is developed with toner and this toner image is transferred onto transfer sheet and then fixed. After that, the transfer sheet is discharged out of the LED printer.
AlGaAs-base materials and the like are generally known as materials for the LEDs of radiative regions for use in combination with this rod lens array.
It is, however, a recent tendency that the resolving power required of the printers is the high resolving power of 600 to 1200 dpi. Under such circumstances, there is such an increasing common tendency that as to a rod lens array employed, a stack of two lines of rod lenses of high resolution type having the nominal angular aperture of 12xc2x0 and the nominal line size of 0.6 mm is used in combination with the LED array.
On the other hand, however, the AlGaAs-base LEDs demonstrate the phenomenon that there often exists a subsidiary (sub) emission band: near 870 nm in addition to a principal (main) emission band near 780 nm, as shown in the spectrum of FIG. 3.
FIG. 3 is a diagram in which the axis of abscissa indicates the wavelength and the axis of ordinate the photosensitive intensity, i.e., how the photosensitive body used can be sensitive to each spectral region by emission intensity of the LEDs.
In the conventional printer heads of low resolution, the dot-to-dot pitch of the rod lens array was sufficiently larger than blur amounts, and thus interference rarely occurred between blurs of dots. Accordingly, the influence of emission of this sub emission band posed no serious problem.
In recent years, however, this sub emission band is coming to affect the image with increases in the resolving power of printers. It is thus extremely difficult to achieve high resolution and high image quality of the printer heads using the AlGaAs-base LED array exhibiting the sub emission band at random.
FIG. 4 shows the imaging relation of an LED radiative point 1 of LED chip 2 with the sub emission band, including LEDs arrayed at the pitch P, through the high-resolution rod lens array 3 of currently well-known type with the nominal angular aperture of 12xc2x0 and with relatively suppressed chromatic aberration. This figure illustrates that the main emission band and the sub emission band demonstrate a small difference D in TC length between TCmain and TCsub, the F-number is also large, and thus the light of the sub emission band is not so blurred on the photosensitive body 4.
FIG. 5 schematically shows how the dots are resolved where wafers with different intensities of the sub emission band are adjacent to each other.
In FIG. 5, the upper part shows a state in which the luminance B of the sub emission band, which varies wafer by wafer across the chip boundary indicated by a dotted line at the center, is superimposed on the luminance A of the main emission band of the constant light intensity, and the middle part schematically shows how a spot image of each LED chip is formed. Consequently, FIG. 5 shows a case in which the sub emission band B affects the spot luminance distribution more on the right side than on the left side. Since the blur of the left sub emission band is small, the sub emission band appears as a light amount unevenness component randomly overlaid on a predetermined development level and thus developed spot sizes vary chip by chip, as seen in the lower part of FIG. 5. As a consequence, the density difference occurs in chip units and it appears as degradation of image quality. Particularly, in the case wherein a wafer chip with different sub emission band characteristics is inserted in a repair step of a chip after die bonding, there appear uneven stripes in the range of several millimeters in a halftone image. This was the drawback of degrading the image quality, particularly, in the case of pictorial imagery.
In addition, it is very difficult to manage the height of the peak of this sub emission band for every wafer in the fabrication process. Further, a method of managing each of these wavelength distributions and carrying out works of the die bonding of chips could greatly affect cost and was not so practical.
An object of the present invention is, therefore, to decrease the influence of the sub emission band, based on the construction of the rod lens array in the LED printer head, provide a configuration in which the light of the sub emission band does not reach the development level, and realize the high image quality.
(Prior Art 2)
In recent years, color office documents are rapidly increasing with spread of personal computers and along therewith the LED printers are drawing attention as printing heads for color printers capable of printing such color documents at high speed. With the conventional LED printers, however, the principal emphasis was on the quality of letters, but emphasis was not laid so much on pictures, halftone images, and so on. In addition, correction for light amounts was also in such a level that variation among chips was corrected by chip resistance.
Therefore, this coming era requires techniques of precisely controlling light amounts while precisely measuring variation of light amounts themselves associated with the imagery, in order to output pictorial color documents.
Meanwhile, for development of high speed printers, the AlGaAs-base materials and the like are generally known as materials for the LEDs enabling highly efficient emission.
The AlGaAs-base LEDs involve the phenomenon that the sub emission band B considered to originate in a GaAs substrate appears in addition to the main emission band A, as illustrated by the solid line in FIG. 10. The wavelength of the main emission band A is approximately 780 nm and the wavelength of the sub emission band B is approximately 870 nm.
It is also common practice to use a silicon PIN photodiode with spectral sensitivity characteristics as indicated by a chain line C in FIG. 10, as a sensor used in measurement of light amounts.
FIG. 11 shows a typical configuration example of a conventional LED-array light-amount measuring device.
This configuration is a typical configuration of measuring apparatus, which is commonly employed by many LED light-amount measuring devices, for example as described in applications filed by the inventor, or in other applications, for example, Japanese Patent Application Laid-Open No. 10-185684.
In FIG. 11, first, an emission signal enough for emission of a light amount to be measured is supplied from a driver 21 of an emission signal generator to the LED array 22 as an object to be measured, to make a predetermined LED emit light. The light emitted travels through an imaging lens 23 to reach a PIN photodiode 26 with the spectral sensitivity indicated by the chain line C of FIG. 10 and a sensor part 24 thereof provides an electric output signal proportional to the light amount. The analog signal of this electric output signal is converted to a digital signal by an A/D converter 25 and a processing system 27 thereafter performs an operation to determine whether the emission amount of the predetermined LED is normal or not.
However, recent research clarified that delicate variation occurred every process of wafer in the light amount of the aforementioned sub emission band B. Therefore, if LED chips cut out of different wafers are ranked by the above method and mounted on a single head, there will occur cases in which chips with different light amounts of the sub emission band B are mixed in the head.
In such cases, since the influence of the light amount of the sub emission band on the actual images was different from that of the main emission band in terms of contribution to sensitivity, there arose the problem that even if the light amounts were measured using the sensor with the spectral characteristics C of FIG. 10 and if ranking of average light amount of chip and correction for light amount of each bit were carried out based on the result of the measurement it was infeasible to match the light amounts with levels of actual images in the situation in which the chips with different sub emission bands were mixed.
An object of the present invention is, therefore, to provide a method of measuring an amount of light from the LED array in the light-amount measuring apparatus for measuring the amount of emission not only from the LED chips but also from the LED array, and to provide an LED printer head and an LED printer fabricated and placed based on the result of measurement by the measuring method.
For solving the problem of prior art 1 described above, a first aspect of the present invention is to fully blur the spot of the sub emission band varying wafer by wafer by making use of the magnitude of axial chromatic aberration between the peak wavelengths of the main emission band and the sub emission band, so as to prevent the light of the sub emission band from reaching the development level, thereby accomplishing the high image quality to the contrary.
The problem of prior art 2 described above was caused because the light amounts of the two emission bands with the different effects on the image were handled on an equal basis.
For solving this problem, a second aspect of the present invention is to interpose an optical element for separating the main emission band from the sub emission band, for example, an optical element with the spectral characteristics D as illustrated in FIG. 6, to separate this main emission band A from the sub emission band B, separately measure and evaluate them, perform an operation according to degrees of influence on the printer, and handle the data as light amount data, thereby enabling accurate correction for light amounts and accurate ranking of chips.
An LED head according to one aspect of the invention is an LED head comprising an LED array of LEDs which emit light according to an image signal and which are arrayed at a resolution pitch P of not less than 600 dpi, and a multi-lens array for forming an emission image of said LED array on an information medium,
wherein each of the LEDs of the LED array has a main emission band being an emission spectrum for formation of a main image and a sub emission band apart from a peak wavelength of the emission spectrum of the main emission band, and
wherein a difference D between best TCs at peak wavelengths of the emission spectrum of the main emission band and an emission spectrum of the sub emission band by the multi-lens array is at least 0.15 mm, and optical adjustment of the LED array and the multi-lens array is implemented so that light of the main emission band is focused in a predetermined imaging relation on the predetermined information medium.
In the LED head according to another aspect of the invention, said information medium is a photosensitive body, the peak wavelength of the main emission band and the peak wavelength of the sub emission band are 50 nm or more apart from each other, and a photosensitive intensity ratio R of the sub emission band to the main emission band in the photosensitive body is not less than 0.01.
In the LED head according to another aspect of the invention, an imaging element satisfying the following relation is used:
(2PF/D)2*R less than 0.01,
where F is an equivalent F-number of said multi-lens array.
In the LED head according to another aspect of the invention, the LED array is AlGaAs-base LED chips.
In the LED head according to another aspect of the invention, the main emission band has a peak in the range of 700 nm to 800 nm and the sub emission band has a peak in the range of 850 nm to 900 nm.
In the LED head according to another aspect of the invention, the multi-lens array is an array of two lines of graded index type glass rod lenses with a nominal angular aperture of 20xc2x0 and a nominal rod size of 0.6 mm in trefoil formation.
An image forming apparatus according to a further aspect of the invention is an image forming apparatus comprising the LED array as set forth in either one of the above LED heads, wherein the information medium is a photosensitive body, the image forming apparatus comprising a developing unit for attaching toner to the photosensitive body to form a toner image thereon, a transfer charger for transferring the toner image formed on the photosensitive body, onto a transfer medium, and a fixing unit for fixing the transferred toner image on the transfer medium.
An image forming apparatus according to a further aspect of the invention is an image forming apparatus comprising the LED array as set forth in either one of the above LED heads, wherein the information medium is a photosensitive body, and the image forming apparatus comprising a printer controller for converting code data supplied from an external device, into an image signal and supplying the image signal to the LED array.
An LED-array light-amount measuring method according to one aspect of the present invention is a method of measuring an amount of light from an LED array, wherein there are provided an LED array of LEDs for an LED head and a sensor portion for receiving an amount of light emitted from an activated LED and generating an electric output corresponding to the amount of light received,
wherein the LED array of a measured object has a main emission band being an emission spectrum for formation of an image and a sub emission band being another emission spectrum apart from a peak wavelength of the emission spectrum of the main emission band,
wherein spectral sensitivity of the sensor portion has approximately flat characteristics to the main emission band and the sub emission band,
wherein an optical element for guiding the light amount of the main emission band with higher efficiency than the light amount of the sub emission band in accordance with sensitivity characteristics of a photosensitive body used with the LEDs is placed between the LED array and the sensor portion and emission characteristics of the LED array are measured.
In the method according to another aspect of the invention, the LED array has the main emission band of the emission spectrum for formation of an image and the sub emission band of another emission spectrum 50 nm or more apart from the peak wavelength of the main emission spectrum and a peak light amount of the sub emission band is 3% or more of a peak light amount of the main emission band.
An LED head according to a further aspect of the invention is an LED head wherein ranking or correction for light amount is effected according to data of measurement of light-amount unevenness of the LED array measured by the method described above.
Another LED-array light-amount measuring method according to a further aspect of the invention is a method of measuring an amount of light from an LED array, wherein there are provided an LED array of LEDs for an LED head and two sensor portions for receiving an amount of light emitted from an activated LED and generating an electric output corresponding to the amount of light received,
wherein the LED array of a measured object has a main emission band being an emission spectrum for formation of image and a sub emission band being another emission spectrum apart from a peak wavelength of the emission spectrum of the main emission band,
wherein spectral sensitivity of the sensor portions has approximately flat characteristics for the main emission band and said sub emission band,
wherein an optical element for reflecting or transmitting a light amount of the main emission band and for transmitting or reflecting a light amount of the sub emission band is placed between the LED array and the two sensor portions,
wherein the light amount of the main emission band is measured by one sensor portion out of the two sensor portions and the light amount of the sub emission band by the other sensor portion, a predetermined operation is carried out over measurement data of the light amount of the main emission band and measurement data of the light amount of the sub emission band to obtain single light-amount measurement data, and emission characteristics of the LED array are measured.
In the method according to another aspect of the invention, the LED array has the main emission band of the emission spectrum for formation of image and the sub emission band of another emission spectrum 50 nm or more apart from the peak wavelength of the main emission spectrum and a peak light amount of the sub emission band is 3% or more of a peak light amount of the main emission band.
In the method according to another aspect of the invention, the predetermined operation is an operation to determine a rate of influence from the main emission band and from the sub emission band according to the sensitivity characteristics of the photosensitive body on which an image is formed according to amounts of light emitted from the LED array and to combine measurement data of the light amount of the main emission band and the light amount of the sub emission band.
Another LED head according to a further aspect of the invention is an LED head wherein ranking or correction for light amount is effected according to data of measurement of light-amount unevenness of the LED array measured by the method described above.
Another LED-array light-amount measuring method according to a further aspect of the invention is a method of measuring an amount of light from an LED array, wherein there are provided an LED array of LEDs for an LED head and a sensor portion for receiving an amount of light emitted from an activated LED and generating an electric output corresponding to the amount of light received,
wherein the LED array of a measured object has a main emission band being an emission spectrum for formation of image and a sub emission band being another emission spectrum apart from a peak wavelength of the emission spectrum of the main emission band,
wherein spectral sensitivity of the sensor portion has approximately flat characteristics to the main emission band and the sub emission band,
wherein an optical element for cutting either a light amount of the sub emission band or a light amount of the main emission band is placed in a retractable state between the LED array and the sensor portion, a predetermined operation is carried out over two output signal values obtained from two states of presence and absence of the optical element from the sensor portion, and emission characteristics of the LED array are measured.
In the method according to another aspect of the invention, the LED array has the main emission band of the emission spectrum for formation of an image and the sub emission band of another emission spectrum 50 nm or more apart from the peak wavelength of the main emission spectrum and a peak light amount of the sub emission band is 3% or more of a peak light amount of the main emission band.
In the method according to another aspect of the invention, the predetermined operation is an operation to determine a rate of influence from the main emission band and from the sub emission band according to the sensitivity characteristics of the photosensitive body on which an image is formed according to amounts of light emitted from the LED array and to combine measurement data of the light amount of the main emission band and the light amount of the sub emission band.
Another LED head according to a further aspect of the invention is an LED head wherein ranking or correction for light amount is effected according to data of measurement of light-amount unevenness of the LED array measured by the method described above.
In the method according to another aspect of the invention, the LED array is AlGaAs-base LED chips.
In the method according to another aspect of the invention, the main emission band has a peak in the range of 600 nm to 800 nm and the sub emission band has a peak in the range of 850 nm to 900 nm.
In the method according to another aspect of the invention, the sensor portion with the flat characteristics is a silicon PIN photodiode.
In the method according to another aspect of the invention, the optical element is a dichroic filter or mirror formed by stacking dielectric films and a medial wavelength of the dichroic filter or mirror is set between the peak wavelength of the main emission band and the peak wavelength of the sub emission band.
In the method according to still another aspect of the invention, the optical element is an absorbing filter having a higher absorption property of the sub emission band than that of said main emission band and a rate of transmittance of the main emission band and transmittance of the sub emission band is approximately equal to a rate of influence on the photosensitive body on which an image is formed according to amounts of light emitted from said LED array, from the light amount of the main emission band and from the light amount of the sub emission band.
In the method according to another aspect of the invention, the absorbing filter is a heat absorbing filter with different absorptances in the main emission band and in the sub emission band and a rate of transmittance of the main emission band and transmittance of the sub emission band is optimized by controlling a thickness of the heat absorbing filter.
Another image forming apparatus according to a further aspect of the invention is an image forming apparatus comprising the LED head as set forth, a photosensitive body, a developing unit for attaching toner onto the photosensitive body to form a toner image thereon, a transfer charger for transferring the toner image formed on the photosensitive body, onto a transfer medium, and a fixing unit for fixing the transferred toner image on the transfer medium.
Another image forming apparatus according to a further aspect of the invention is an image forming apparatus comprising the LED head as set forth, and a controller for converting code data supplied from an external device, into an image signal and supplying the image signal to the LED array.